I think this is a good example that we can all learn from. Everybody has habit or routines. Some of them good, some of them bad. Sometimes you don't realize you have a bad habit until something like this. So the lessons to learn are
1. Nobody is infallible. We are all capable of making mistakes, despite are best intentions and despite how good of a routine/plan/system we think we have.
2. It doesn't hurt to double check. Or triple check.
3. Complacency causes problems.
I bet many people don't always check their carry firearm. It's the firearm you carry, you know it is loaded, you carry it all the time, I just carried it, so today is no different. It's loaded. Until the one day that is out of routine and it isn't. I've been there. Had I not checked I might not have noticed. It would have been super easy to be a bit complacent and not check. As it was, when I checked and realized it wasn't loaded I was surprised. My point? When you check the same thing over and over, and it is always loaded, you are never surprised. When you are never surprised it becomes easy to become complacent. Not until you have an instance where you are surprised like this do you really understand what that means.